A. Technical Field
Fine resolution device and circuits are fabricated by one or a series of steps each involving lithographic resolution followed by selective treatment of regions of a layer of initially continuous, homogeneous device material. Lithography is ordinarily carried out in "actinic" material which is subsequently developed to result in aperture-delineation. Such patterns serve to directly or indirectly mask material undergoing etching or other processing. Selective etching may be carried out by dry processing for example, by means of a species produced in a plasma to wet processing where particularly fine resolution is desired.
Such fabrication is used in production of Large Scale Integrated silicon circuitry. Use is contemplated as well for discrete devices, in other semiconductor technology, for integrated optical circuitry, for magnetic memories, etc.
B. History
Large Scale Integrated circuitry as well as other high resolution planar structures are generally fabricated through a series of levels. One procedure, common to construction of most such circuits involves first producing a masking layer within a continuous region of actinic material by selective exposure to radiation followed by solution development to selectively remove material which has been rendered either more or less soluble through irradiation. Such masking layers have served as discrete masks (sometimes with the additional step of replication of the pattern in an underlying layer of some more durable layer such as chromium).
This mask technology now in prevalent use in the fabrication of silicon integrated circuits has undergone considerable development to the present point at which design rules of a few microns are regularly attainable. Discret masks so used serve for secondary delineation of patterns in expendable photoresist layers which are developed to serve for actual device processing. Photoresist layers are removed at each processing level to permit fabrication at the next level.
It is generally believed that mask technology will be superceded by a maskless technology (direct processing) to produce finer resolution and higher device density. In accordance with such contemplated procedures, primary rather than secondary delineation will be in expendable resist layers tightly adherent to the device undergoing processing. Such resist layers may be true photoresists or may use short wavelength radiation.
Regardless of procedures--whether mask or maskless; regardless of involved technology, a procedure common to all such fabrication involves selective etching of continuous layers of device-functional material. To date, wet etching--for example by use of aqueous acid media--has found satisfactory use. As resolution needs become more stringent, inherent limitations become more significant. Liquid media reacting with polycrystalline or amorphous layers together result in isotropic etching. Resulting undercutting, (removal of material under the masking material) imposes a limit on spacing.
Increasing miniaturization has resulted in appreciation for advantages of dry processing. Etching by momentum transfer, for example by ion milling, imparts, directionality to material removal and eliminates undercutting. High accelerating fields resulting in energetic particle bombardment of surfaces being processed sometimes causes a new set of problems. Lattice damage at some levels of fabrication may even destroy the device.
At the other end of the spectrum, dry processing may depend upon plasma assisted reactions. Plasma etching, for example, is dependent upon removal primarily due to chemical reaction of material to be removed with plasma-produced etching species. As in momentum transfer processing, etched product may be easily removed--in this instance by system selection to result in vapor state reaction product. Plasma assisted etching, however, may be isotropic in behavior. Anisotropy may result from use of large plasma fields and low pressures but this may, in turn, produce intolerable lattice damage as well as rapid resist erosion. As in wet etching, imperfect end point detection complicated by unequal wafer-to-wafer etching may result not only in extreme undercutting, but also in etching of underlying layers. The latter is alleviated by etching systems with pronounced selectivity for material being etched relative to underlying material.
A variety of materials are encountered in LSI production. For present design rules oxide layers are satisfactorily plasma etched with mixtures of methane and oxygen. The same mixtures are applied to many other materials but selectivity is generally poor. Aluminum or aluminum-rich layers are etched by plasma species resulting from introduction of carbon tetrachloride. CCl.sub.4, a liquid at room temperature, is difficult to monitor. Heating to increase volatility results in unwanted condensation in cool regions.
A recognized problem with CCl.sub.4 --unreliable initial etching has been solved by use of BCl.sub.3 (see J. Vac. Sci. Technol., 14, No. 1 p. 266 1977) A continuing problem in aluminum etching is poor discrimination--due to significant etching of SiO.sub.2 as well as Si. A further problem, attributed to unwanted polymer formation, is evidenced by unwanted etching following exposure to the atmosphere. It is believed that difficulty removed polymer, reacts with atmospheric moisture to produce HCl which is responsible for continuing etching.